Modern weapons systems rely heavily on the use of electronics to provide system operators with increased control capabilities. An example of this is a tube-launched, optically tracked wire-guided anti-tank missile such as the TOW missile developed by the Hughes Aircraft Corporation for the U.S. Army in the 1960's.
Guided missiles of this type fly to the point indicated in the crosshairs of a launcher's sight within a design range of the missile, e.g. 3,750 meters in case of a TOW missile. The tracking system optically tracks a flare originating from the back of the missile, e.g. in the infra-red range, and sends guidance signals through two fine steel wires attached to and dispensed from the missile during its flight. The tracking system may be mounted on the missile launcher tube and its major components are a trackable optical sight, a missile tracker and a guidance set. A gunner tracks the target by keeping the crosshairs of the optical sight on the target. The missile tracker senses the optical, e.g. infra-red flare on the missile and generates signals representing the missile's deviation from the gunner's line-of-sight in the horizontal and vertical plane. This deviation appears in the form of error signals which are processed by a feedback control loop in the guidance set. The guidance set produces a correction guidance signal which is sent to the missile by the wire link and the missile responds to these signals by servomechanism deflection of control fin surfaces located at the rear of the missile.
In the following the invention will at times be described with reference to the TOW missile, it being understood that it is not confined thereto.
The TOW missile system uses a command to line-of-sight guidance concept. The function of the guidance set is to minimize missile trajectory deviations due to error sources such as gunner jitter, tracker and sensor noise, gusts and cross-winds, flight motor thrust axis misalignments, system imbalances, variations in launch conditions, etc.
Since its development, the TOW missile system has seen combat experience which has provided important information as to its overall utility. While the missile itself is capable of a range of 3,750 meters, the original guidance set was not designed to effectively utilize the maximum range. The need to utilize the missile at its maximum range is apparent, especially in regard to developments which have increased the firing range of new tanks. Investigation of the hit probabilities for the existing TOW missile system have revealed that the chance of a target hit at a range of 3,500 meters is 73.5% for a good gunner. For a poor gunner, the chance of a target hit for a range of 3,500 meters is only 7.5%. Thus, it would be desirable to improve the performance of the original TOW missile system guidance set so as to exploit the longer range of the basic TOW missile.
An analysis into the performance of the existing guidance set was performed and the following was revealed:
1. The inherent TOW missile construction provides low aerodynamic damping which makes it highly susceptible to the effects of aerodynamic forces, resulting in fluctuations during its flight. PA1 2. Over the longer range of flight, the gunner must maintain his sight on the target for a longer period of time. The feedback control system depends on the steadiness and the aiming ability of the gunner. PA1 3. During flight internal inertial forces are relied upon to guide the missile to its target, and initially introduced errors in its trajectory grow over time. The control loop is thus required for reconciling the need to correct larger errors as time goes on, with the fact that with longer flight time, i.e. with an increase of the error gain, the control loop may become unstable. PA1 4. Large oscillations of the missile around trim position create a decrease in missile speed through increased wind resistance.
The above summarizes the limitations of the existing TOW missile system with regard to exploitation of the maximum missile range. It is accordingly an object of the present invention to improve the guidance set of a tube-launched, optically tracked, wire-guided missile such as, for example, the TOW missile and to achieve thereby an increased target hit probability over the maximum missile range.
Tube-launched missiles of the kind specified comprise a flight motor which expires a few seconds after launching and a relatively long time before the missile hits its target. In the flight control of a tube-launched missile of the kind specified it is accordingly necessary to ensure that the missile flies along the desired flight trajectory, usually close to the ground surface, and avoid premature landing even after the expiry of the flight motor, and to this end a so-called gravity bias input is applied to one of the control loop stages. By this input the angle of attack of the missile is caused to vary with time in synchronism with the gradual reduction of the missile velocity thereby insuring that the missile maintains a desired lift also after the flight motor has expired. In order to achieve this, the gravity bias applied to the flight control has to be designed so as to take into account the change of the velocity in time. The rate of reduction of velocity is a function of the weight and geometry of the missile.
The gravity bias further ensures that the missile keeps a predetermined trajectory during the first few seconds after launching when sighting of the target is impossible due to smoke and dust created during launching.
Modern battle conditions often require the use of two or more different missiles having the same electronic guidance system but differing from each other in their weight and/or geometry. The electronic control system including the gravity bias is built into the launcher and consequently in accordance with the state of the art, where different types of missiles with similar electronic controls but requiring different gravity biases are to be launched from a given site, different launchers are required for each type of missile. This constraint is burdensome and onerous, in particular where great mobility and simplicity of operation are required and it is therefore another object of the present invention to provide a multi-purpose launcher for tube-launched, optically tracked wire-guided missiles adapted for the launching of two or more different missiles having the same electronic control system but having different physical characteristics (weight and geometry) and therefore requiring different gravity bias values.
Once rolling of the missile about its longitudinal axis has stopped, two types of corrections are required for the flight control of a TOW missile, namely the correction of the elevation pitch and the correction of the azimuth or yaw, and for this purpose two separate channels are needed. In conventional TOW missiles and two channels have identical time variable gains.